1. Field of the Invention
The present invention refers to a liquid crystal display and drive method thereof, and particularly to a TFT active matrix display and drive method thereof for low power consumption.
2. Related Background Art
A prior art liquid crystal display is disclosed, for example, as a liquid crystal display to get a high definition display image in the Official Gazette of Japanese Patent Laid-Open NO. 133629/1998 discloses. Another example is disclosed in the Official Gazette of Japanese Patent Laid-Open NO. 113876/1997 where a polarity inversion circuit is connected to an opposite electrode to ensure stable operation and low power loss. The Official Gazette of Japanese Patent Laid-Open NO.104246/1995 discloses an active matrix liquid crystal drive for lower power consumption.
The following describes the prior art TFT active matrix drive system:
A line sequential scanning system is used to drive the TFT active matrix liquid crystal display, and one scanning pulse is applied to each scanning electrode for each frame time. About 1/60 second is appropriate as one frame time. These pulses are applied downwardly from the top of the panel to the bottom in sequence at differently timed intervals. Consequently, 480 gate wires are scanned in one frame in a liquid crystal display having a 640xc3x97480-dot pixel configuration, so the time range of the scanning pulse is about 35 microseconds.
Meanwhile, the liquid crystal drive voltages applied to liquid crystals for one-row pixels where scanning pulses are applied are simultaneously applied to the signal electrode in synchronization with scanning pulses. In the selection pixel where gate pulses are applied, the gate electrode voltage of the TFT connected to the scanning electrode is increased to turn on the TFT. In this case, liquid crystal drive voltage is applied the display electrode via the source and drain of the TFT to charge the pixel capacity comprising the liquid crystal capacity formed between the display electrode and opposite electrode formed on the opposite substrate, plus load capacity assigned to the pixel. This operation is repeated to allow liquid crystal application voltage to be applied repeatedly to the pixel capacity of the all panel surfaces for each frame time.
Since a.c. voltage is required to drive the liquid crystal, the voltage with the polarity inverted for each frame time is applied to the signal electrode. As a result, even if the image to be displayed does not change, much of the power to drive the panel is consumed to repeatedly charge or discharge, at every gate selection, the capacity at the crossing portion between scanning and signal lines or the capacity of the liquid crystal between the line and the opposite electrode formed on all surfaces of on the opposite substrate.
The Official Gazette of Japanese Patent Laid-Open NO.258168/1997 discloses a technology to solve said problem and to implement a liquid crystal display of lower power consumption.
The liquid crystal display disclosed in the Official Gazette of Japanese Patent Laid-Open NO.258168/1997 has the following components in each of the pixel areas enclosed by multiple scanning electrodes and multiple signal electrodes of a substrate; (1) a display data retention circuit connected to corresponding scanning electrodes and signal electrodes to capture and retain the display data from signal electrodes in response to scanning signals, (2) a switching element connected to the display data retention circuit wherein switching operation is controlled by said circuit, and (3) a display electrode connected to the switching element. Display electrode drive voltage is changed in response to the data retained by the display data retention circuit, thereby controlling pixel indications.
The display data retention circuit has a sampling TFT where the gate is connected to the corresponding scanning electrode and the drain is connected to the corresponding electrode, and a sampling capacitor connected to the sampling TFT source. The switching element has a switching TFT where the gate is connected to the source of the display data retention circuit and the source is connected to said display electrode. The sampling capacitor comprising said display data retention circuit and the drain of the switching TFT connected to the display electrode are connected to the common electrode.
The display data retention circuit sends to the sampling capacitor via the sampling TFT the display data signal voltage fed from the signal electrode in synchronism with the scanning signal to select the scanning electrode, and retains the pixel display data as voltage information.
The liquid crystal drive voltage controlling the light and dark pattern of the pixel is determined by a.c. voltage applied to the liquid crystal held closely between the display electrode and opposite electrode. When liquid crystal drive power voltage is applied to the opposite electrode, the voltage is applied to the liquid crystal if the switching TFT is on, but not applied to the liquid crystal if said switching TFT is off. This arrangement allows liquid crystal applied voltage of each pixel to be controlled by the display data signal voltage in the pixel.
In this case, the display data retention circuit can continue to retain the display data until voltage across the sampling capacitor as display data signal voltage is discharged below the threshold voltage of the switching TFT due to leakage of switching TFT or the like. Time until said discharge occurs depends on the leakage current value of the switching TFT and the capacity of the sampling capacitor. Normally, the TFT leakage current value is very small, but is sufficiently longer than 16.6 msxe2x80x94a representative value of frame time. Moreover, liquid crystal drive voltage can be applied to all pixels in one operation from the opposite electrode. For pixels where display contents do not change, display can be maintained by application of liquid crystal drive voltage alone if the display data signal voltage is changed and the switching TFT is turned on or off. Scanning signal and display data signal voltage should be applied only when display contents are to be rewritten. This ensures excellent display while keeping low power consumption inside the panel.
However, said prior art has a problem that much is required to rewrite the image in response to changes of display contents.
Voltage across the sampling capacitor changes in response to changes of display contents, and this involves changes in the state of the switching TFT. In this case, if the switching TFT changes from OFF to ON state, the voltage of the display electrode will become the same as that of the common electrode immediately. Voltage will be applied to the liquid crystal to get the desired display.
However, if switching TFT is changed from the ON to OFF state, the display electrode is in the floating mode while voltage between the display electrode and opposite electrode is retained, so d.c. voltage will be applied to the liquid crystal between the display electrode and opposite electrode. The desired display cannot be obtained. This d.c. voltage is reduced by liquid crystal leakage, but the time constant for this reduction is long. Complete switching takes much time.
Although the TFT leakage current is very small, it is not zero. The voltage stored in the sampling capacitor cannot be retained for a long time. This makes it necessary to make up for the voltage reduced by leakage whenever required, even if there is no change in display contents. In other words, overwriting is sometimes necessary. When overwriting, the voltage of the sampling capacitor is changed by making up for it. However, if this change affects the state of the switching TFT, the image will change; this is not preferred. In other words, this requires the sampling capacitor voltage to be overwritten without changing the state of the switching TFT.
When overwriting, pulse signals are normally applied to the scanning electrode, and voltage corresponding to the display of pixels for one row is applied to the signal electrode in one operation in synchronism with pulse signals. In this case, a latch circuit is required to output the synchronized voltage to the signal electrode. If the drive circuits of the signal electrode and scanning electrode are built in the liquid crystal panel using a polysilicon or the like, it is preferred to omit the use of the latch circuit, thereby reducing the circuit size. In this case, the voltage of the scanning electrode in the corresponding row is reduced below the threshold value of the sampling TFT, and the signal electrode voltage is rewritten into voltage corresponding to the display for the row. However, the following operation error will occur in this case.
According to the method where the latch circuit, voltage corresponding to the display of pixels on the same column of the preceding row remains in the signal electrode when the voltage of the scanning electrode is reduced below the threshold value of the sampling TFT. Consequently, the data corresponding to pixels on the same column of the preceding row will be written into the sampling capacitor. Normally, the desired data are written immediately thereafter, so there is no problem. If the display data in the same column of the preceding row is on, and the display data to be written is off, then an operation error will occur.
Namely, the switching TFT changes from ON to OFF state with a.c. voltage applied to the liquid crystal. So d.c. voltage is applied to the liquid crystal between the display electrode and opposite electrode as described above, with the result that the desired display cannot be obtained.
According to said technology, the switching TFT may be turned off depending on the screen to be displayed. Since the power of the liquid crystal display is turned on, for example, unwanted d.c. voltage having occurred when power is turned on will remain applied to the electrode of the pixel where the switching TFT is off. If the pixel electrode is always kept is in the floating mode during the drive, the voltage will become unstable. This is not to be preferred.
Problems described above are unique to said technology where display is given with the pixel electrode kept in the floating mode. Such problems do not occur in the prior art technologies disclosed in the Official Gazette of Japanese Patent Laid-Open NO.133629/1998, Official Gazette of Japanese Patent Laid-Open NO.113876/1997 and Official Gazette of Japanese Patent Laid-Open NO.104246/1997 where switching elements are not used.
The object of the present invention is to provide a liquid crystal display and drive method thereof, featuring a lower power consumption and high speed display switching, based on the method where display is performed with the pixel electrode kept in the floating mode.
Another object of the present invention is to provide a liquid crystal display and drive method thereof, featuring a lower power consumption and high speed display switching, based on the method where display is performed with the pixel electrode kept in the floating mode; said liquid crystal display further characterized by a simple circuit configuration and a function of preventing d.c. voltage from being applied to the liquid crystal when the switching TFT is changed from ON to OFF state.
Still another object of the present invention is to provide a liquid crystal display and drive method thereof, featuring a lower power consumption and high speed display switching, based on the method where display is performed with the pixel electrode kept in the floating mode; said liquid crystal display further characterized by a function of preventing d.c. voltage from being applied to the liquid crystal of the pixel where the switching TFT is always off.
The present invention is characterized a liquid crystal display comprising (1) a switching element connected to a display data retention circuit, common electrode and display electrode, said switching element controlling said common electrode and said display electrode according to the voltage retained in said display data retention circuit, and (2) an opposite electrode installed opposite to said display electrode where a.c. voltage vibrating in response to the voltage of said common electrode is applied; wherein display is performed based on the fact that a.c., voltage is applied to a liquid crystal layer when said switching element connects between said display electrode and common electrode, and a.c. voltage is not applied to said liquid crystal layer when said switching element releases connection between said display electrode and said common electrode; said liquid crystal display further characterized in that the state of said switching element is changed from connection between said display electrode and said common electrode to release of said connection, when the voltages of said opposite electrode, said display electrode and said common electrode are made substantially the same by stopping said a.c. voltage applied to said opposite electrode.
The liquid crystal display according to the present invention has the following components in each of the pixel areas enclosed by multiple scanning electrodes and multiple signal electrodes of a substrate; (1) a display data retention circuit connected to corresponding scanning electrodes and signal electrodes to capture and retain the display data from signal electrodes in response to scanning signals, (2) a switching element connected to the display data retention circuit wherein switching operation is controlled by said circuit, and (3) a display electrode connected to the switching element. Display electrode voltage is changed in response to the data retained by the display data retention circuit, thereby controlling pixel indications.
The display data retention circuit has a sampling TFT where the gate is connected to the corresponding scanning electrode and the drain is connected to the corresponding signal electrode, and a sampling capacitor connected to the sampling TFT source. The switching element has a switching TFT where the gate is connected to the source of the sampling TFT of the display data retention circuit and the source is connected to said display electrode. The sampling capacitor comprising said display data retention circuit and the switching TFT connected to the display electrode are connected to the common electrode.
The display data retention circuit captures into the sampling capacitor and retains therein the display data signal voltage fed from the corresponding signal electrode by making the voltage of the corresponding scanning electrode equal to or greater than the threshold value of the sampling TFT. This operation is repeated by scanning row by row to write display data to all pixels. The liquid crystal drive voltage controlling the light and dark pattern of the pixel is determined by a.c. voltage applied to the liquid crystal held closely between the display electrode and opposite electrode.
When liquid crystal drive power voltage is applied to the opposite electrode, the voltage is applied to the liquid crystal if the switching TFT is on, but not applied to the liquid crystal if said switching TFT is off.
The liquid crystal display according to the present invention is characterized in that voltages of the opposite electrode and display electrode are made substantially the same as that of the common electrode when the switching TFT is changed from ON to OFF state. In this case, voltages of the display electrode voltage and common electrode are the same with each other when the switching TFT is on. Consequently, voltages of these three components are made substantially the same if the opposite electrode voltage is made substantially the same as those of the display electrode and common electrode. Said expression xe2x80x9cVoltages of these three components are made substantially the samexe2x80x9d also means that the voltage applied to the liquid crystal layer, namely, the difference of voltages between the opposite electrode and display electrode (and common electrode) is made not to exceed the threshold value, in addition to the fact that the opposite electrode voltage is made the same as the display electrode voltage (and common electrode voltage).
As described above, if the opposite electrode and display electrode voltages are made substantially the same as that of the common electrode when the switching TFT is changed from ON to OFF state, the display electrode voltage is the same as the common electrode voltage, even if the switching TFT is changed from ON to OFF to keep the display electrode in the floating mode. So d.c. voltage is not applied to the liquid crystal as discussed in the above description of problems.
Drive is given to ensure data of the data retention circuit is rewritten without voltage applied to the liquid crystal, by making the voltages of the opposite electrode and display electrode the same as that of the common electrode when display has switched. As a result, voltage applied to the liquid crystal is zero, even if the switching TFT is changed from ON to OFF state to keep the display electrode in the floating mode. So d.c. voltage is not applied to the liquid crystal as discussed in the above description of problems. If a.c. voltage is applied to the opposite electrode after all data have been rewritten, a.c. voltage is applied to the liquid crystal where the switching TFT is on, and no voltage is applied to the liquid crystal where the switching TFT is off. Then a desired display is selected.
Another type of the liquid crystal display according to the present invention uses a circuit which turns off all switching TFTs after display electrode voltages in all pixel areas are simultaneously made the same as common electrode voltage. When display is switched, switching TFTs are turned off after display electrode voltages in all pixel areas are made the same as common electrode voltage. Under this condition, data stored in the display data retention circuit are rewritten. In this case, the state of the switching TFT is changed while a.c. voltage is applied to the liquid crystal. All switching TFTs are off before data are rewritten. During data rewriting, the state does not change from ON to OFF. In other words, this eliminates the possibility of the problem which may occur when the switching TFT is changed from ON to OFF state.
When the display data of the display data retention circuit is rewritten with a.c. voltage applied to the liquid crystal, or the same display data is overwritten with a.c. voltage applied to the liquid crystal in order to make up for the voltage stored in the sampling capacitor reduced by leakage, data corresponding to the pixel in the same column of the preceding row may be written, if the scanning electrode has reached the threshold value or has exceeded it while the display data signal voltage corresponding to the pixel in the same column of the preceding row still remains in the signal electrode. Normally, the desired data are written immediately thereafter, so there is no problem. If the display data on the same column of the preceding row is on, and the display data to be written is off, then said problem will occur. Namely, the switching TFT changes from ON to OFF state with a.c. voltage applied to the liquid crystal. So d.c. voltage is applied to the liquid crystal as described above, with the result that the desired display cannot be obtained.
To solve this problem, still another type of the liquid crystal display according to the present invention has a latch circuit installed to the signal data write circuit to synchronize the scanning electrode voltage with signal electrode voltage. This ensures that voltage not exceeding the threshold value of the sampling TFT will not be applied to the scanning electrode, when the data of the preceding row remains in the signal electrode.
However, installation of a latch circuit increases the circuit size of the signal data write circuit, so this is not appropriate when the circuit is built in the liquid crystal panel using polysilicon or the like. To solve this problem, the present invention proposes a method which does not use a circuit; a method of resetting the signal electrode voltage to the OFF display data signal voltage for each writing into one row. This ensures all signal electrode voltages are OFF display signal voltage when the scanning electrode voltage is equal to or greater than the threshold value of the sampling TFT, so all the switching TFTs of that row will be turned off. If the original state is on in this case, the state will change from ON to OFF, said problem will occur. However, xe2x80x9cONxe2x80x9d will be written immediately thereafter and d.c. voltage is applied only momentarily, so there is no problem.
Another method of solving this problem according to other characteristic of the present invention without installing a latch circuit provides a driving scheme which makes the scanning electrode voltage equal to or greater than the threshold value of the sampling TFT after desired display data signal voltages have been written to all signal electrodes. Furthermore, said problem can be solved, without installation of a latch circuit, by a drive scheme of making the opposite electrode voltage equal to the common electrode voltage at the time of rewriting and overwriting.
The liquid crystal display according to the present invention reduces power consumption by reducing the time period of rewriting or overwriting the display data of the display data retention circuit. The present invention provides a liquid crystal display which reduces the time period of rewriting or overwriting the display data by inputting the address data of the black or white display pixel, instead of inputting the display data corresponding to all pixels.
A further type of the liquid crystal display according to the present invention has a circuit which turns off the switching TFT in said pixel area for at least one row after the display electrode voltages in the pixel area for at least one row are simultaneously made equal to the common electrode voltage. When data is written into the display data retention circuit, the switching TFT is turned off after the display electrode voltage in said pixel area for at least one row is made equal to the common electrode voltage. In that state, data are written to the display data retention circuit in the pixel area for at least one row. In this case, the state of the switching TFT is changed while a.c. voltage is applied to the liquid crystal. The switching TFT is off before data is rewritten. During data rewriting, the state does not change from ON to OFF. In other words, this eliminates the possibility of the problem which may occur when the switching TFT is changed from ON to OFF state. The above operations are performed for all rows and data are written to data retention circuits in all pixel areas. As described above, driving the liquid crystal display allows all display electrodes to be electrically connected with the common electrode every time data is always written to the corresponding display data retention circuit. This eliminates the possibility of the problem of d.c. voltage which may occur the switching TFT based on said technology is off.